1. Field of the Invention
The present invention relates to a suspension apparatus of a vehicle.
2. Description of Related Art
A suspension apparatus of a vehicle, as is generally called a passive suspension, has a damper unit comprising a hydraulic damper and a sprinq (generally a coil spring), and a suspension characteristic is equally set on the basis of a predetermined characteristic of the damper unit. Although the damping force of the hydraulic damper can be variable, this cannot largely change the suspension characteristic.
Recently, a suspension apparatus, as called an active suspension, has been proposed, which allows a change in the suspension characteristic in an arbitrary fashion. This active suspension is basically designed so as to control the suspension characteristic by controlling a supply and a discharge of operating liquid to or from a cylinder unit interposed between a sprang weight and an unsprung weight.
In the active suspension, the suspension characteristic can be changed to a great extent by a variety of control such as ride height control, roll control, pitch control, etc. And for the active suspension, a ride height sensor for detecting a ride height of the vehicle body is used basically for posture control.
It is noted herein that the vehicle with an automatic transmission may cause pitching, as a so-called squatting phenomenon, upon shifting range positions from its rest position to driving position. For instance, for the vehicle of the rear wheel drive type, when the speed range positions are shifted from the parking range position P or neutral range Position N to the driving range position D, torque of the engine is transmitted to cause the rear wheels to advance while the vehicle body is prone to staying at its rest position due to its large inertia, thereby causing the height of the rearward portion of the vehicle body to become lower while the height of the forward portion thereof to raise, i.e., the squatting phenomenon. The degree of this squatting phenomenon is represented as a pitch angle indicating an angle of inclination of the vehicle body. It is therefore considered that this active suspension unit be incorporated into the vehicle with the automatic transmission, with the attempt to prevent an occurrence of the pitching phenomenon likely to be caused resulting from shifting the range positions as described hereinabove by means of the pitch control by the active suspension unit. This pitch control however, is operated so as to control the pitching after it has actually occurred, so that there should be a delay in operation of this unit in response to the actual occurrence of the pitching phenomenon. Therefore, the pitching resulting from the shifting of the range positions cannot be controlled to a sufficient extent.
Further, when the brake is operated to suspend the vehicle body, there may occur a pitch, as a so-called nose-diving phenomenon, which is a phenomenon in which the vehicle is forced to stop during running, while the vehicle is prone to still advancing forward due to its inertia. As a result, temporarily, the height of the forward portion of the vehicle body is lowered while the height of the rearward portion of the body is raised to a higher position. A degree of this nose-diving phenomenon may be represented, for example, by a pitch angle indicating an angle of inclination of the body. In this case, too, control over this pitching phenomenon by the active suspension unit may cause some delay in responding to an actual occurrence of the pitch, so that this control cannot sufficiently block the pitching resulting from operation of the brake.
Furthermore, when the steering wheel is operated from the straight running to the left, for example, the vehicle body is caused to roll to the right due to transverse acceleration, thereby lowering the right-hand side portion of the body yet raising the left-hand side portion thereof. A degree of this roll may be represented as a roll angle (or a roll amount) indicating an angle of inclination of the vehicle body in its transverse direction. In using the active suspension unit for control over a roll, i.e., roll control, this control is performed first by determining an actual roll amount on the basis of the ride height of the body on its left side portion and the ride height of the body on its right side portion and changing the actual roll amount so as to reach a target roll amount. Therefore, there should occur a delay in performing the roll control after occurrence of this roll. Therefore, when the steering wheel is rapidly steered and the transverse acceleration is rapidly changed, too, a considerable delay may occur until the actual roll amount is converged to its target roll amount. This rapid operation of the steering wheel may cause a temporary roll to happen.
In summary, as described hereinabove, when driving operation is performed to such an extent that a status amount of the posture of the vehicle body (hereinafter sometimes referred to as a posture status amount), such as a pitch amount or a roll amount of the body, is changed, a considerably large extent of changes in the posture of the vehicle body cannot be avoided inevitably from such driving operation due to a delay in control of the active suspension from the change in the posture of the body.